As a sensor for verifying that a motor has stopped rotating, there is, for example one, which uses an electromagnetic pickup (refer to PCT Publication No. WO94/23303 (PCT/JP93/00411)).
With this device, a metal disc having a plurality of metallic protrusions located at even spacing around the surface periphery thereof is secured to a motor rotation shaft, and an electromagnetic pick up, serving as a sensor head, is provided at a location facing the protrusions at a predetermined distance therefrom. When the metal disc rotates with rotation of the motor, fluctuations are produced in a magnetic flux with the passing of the respective protrusion. By subjecting the resultant fluctuations in the output from the electromagnetic pick up, due to these magnetic flux fluctuations, to threshold value processing or the like, using a signal processing circuit, then during rotation of the motor (including rotation under inertia), an output of logic value 0 showing that the motor is rotating (danger side) is produced from the signal processing circuit, while when the motor rotation has practically stopped (safe side), an output of logic value 1 is produced from the signal processing circuit.
However, in devices which use an electromagnetic pickup for the sensor head, the spacing between the protrusions and the electromagnetic pick up is extremely small. Moreover, this spacing must be controlled very accurately. Therefore, the sensor head attachment operation becomes difficult. Furthermore, there is a problem in that the fitted location of the pickup head and related parts must be periodically checked, thus increasing maintenance and inspection costs.
Moreover, the circuit configuration for prohibiting generation of a motor stopped judgment output, for the period including from after shutting off the motor power source until motor rotation under inertia has reduced to a sufficiently slow speed is complicated.
The present invention takes into consideration the abovementioned situation, with the object of providing a motor stopped verification sensor which does not have an electromagnetic pickup type sensor head, thus obviating the need for control of the attachment position, and hence the attachment operation is extremely simple.